1. Field of the Invention
The present invention relates to a decoder for an optical communication system that uses code division multiplexing (CDM), more particularly to a parallel decoder.
2. Description of the Related Art
The following documents are referred to below:
Non-patent document 1: Tamai et al., ‘Jisedai hikari akusesu, shisutemu COF-PON no kenkyu kaihatsu’ (Research and development of COF-PON: a next-generation optical access system), Oki Technical Review, Issue 210, Vol. 74, No. 2, April 2007
Non-patent document 2: Kashima et al., ‘Ko-QoS maruchi media hikari haishin shisutemu no kenkyu kaihatsu—COF transhiba’ (Research and development of high-QoS multimedia optical distribution system—COF transceiver) Oki Technical Review, Issue 200, Vol. 71, No. 4, October 2004
Non-patent document 3: Sasase, ‘Hikari shisutemu ni okeru hikari fugo bunkatsu tagen setsuzoku gijutsu’ (Optical Code Division Multiple Access Techniques in Optical Communication Systems) TELECOMFRONTIER, November 2004
Non-patent document 4: T. Sugiyama et al., ‘HEMT CCD MF for Spread Spectrum Communication’, 6th Topical Workshop on Heterostructure Microelectronics (TWHM 2005) August 2005
Patent document 1: Sasaki et al., Japanese Patent Application Publication No. 2003-317026, Signed product sum computing element and analog matched filter including the same, Nov. 7, 2003
Passive optical network (PON) systems that use CDM for communication between a provider and users via optical fibers are attracting attention. Known as CDM-on-fiber-PON or COF-PON systems, these systems permit transmission over longer distances than are feasible in more traditional time division multiplexing (TDM) optical access systems. COF-PON also has the advantage of being compatible with wavelength division multiplexing (WDM). A WDM-CDM-PON system is described in non-patent document 1, for example.
Current demands in the optical access network field include faster data transmission rates and higher degrees of multiplexing. COF-PON has natural multiplexing advantages, because it transmits multiple channels bi-directionally over the same wavelength. For high data transmission rates, however, COF-PON requires components that can operate at very high speeds, because each data bit is transmitted not as a single bit but as a sequence of very short chips.
In particular, a COF-PON system requires decoders with matched filters that can operate at the chip rate in order to correlate the received signal with a code signal to recover the transmitted data bits. At present, analog matched filters are used in COF-PON decoder circuits, as disclosed in non-patent documents 1-3 and patent document 1 above, for example, because analog matched filters are superior to digital matched filters in terms of operating speed, device size, and power consumption. In the future, if faster, smaller, lower-power digital matched filters become available, they may replace analog matched filters because they are easier to manufacture and can be produced in high volumes at a low cost.
At present, COF-PON systems have achieved data transmission rates of about one hundred megabits per second per user at a chip rate of about two gigachips per second. These systems employ analog matched filters operating at a clock frequency of about two gigahertz, which represents the limit of current analog matched filter technology, as described in non-patent document 4 above. One of the factors that limits analog matched filter operating speed in a COF-PON system is poor charge transfer efficiency due to substantial residual charge in the long shift register that is needed to accommodate the long codes that must be used in order to multiplex a large number of channels.
COF-PON systems employ spreading codes in which the entire code length is used to spread each bit into a sequence of chips, so that the chip rate is the product of the bit rate and the code length, and the code length is equal to the coding gain.
It would be highly desirable to provide higher data transmission rates with existing analog matched filter technology. Through diligent research, the present inventor found that this is can be done by means of a decoder having two analog filters that operate in parallel.